Prekallikrein (PPK) and total kininogen (KGN) were determined in human plasma. PPK was determined by radiochemical esterolytic assay, KGN by radioimmunoassay of bradykinin after trypsin digestion of plasma. In normal volunteers, no changes in PPK or KGN were effected by changes of posture, by infusions of voasoactive amines, or by variation of dietary sodium. Patients with essential hypertension and with Bartter's Syndrome have normal values of PPK, but may have supranormal values of KGN. In a patient with renal artery stenosis, KGN values from the affects side were lower than values from the normal kidney, with PPK values equal.